lnonion: add helper properties minor refactoring

diff --git a/electrum/lnonion.py b/electrum/lnonion.py
index a2f1a56..06b500f 100644
--- a/electrum/lnonion.py
+++ b/electrum/lnonion.py
@@ -25,6 +25,7 @@
 
 import io
 import hashlib
+from functools import cached_property
 from typing import Sequence, List, Tuple, NamedTuple, TYPE_CHECKING, Dict, Any, Optional, Union, Mapping
 from enum import IntEnum
 from dataclasses import dataclass, field, replace
@@ -157,6 +158,10 @@ class OnionPacket:
             version=b[0],
         )
 
+    @cached_property
+    def onion_hash(self) -> bytes:
+        return sha256(self.to_bytes())
+
 
 def get_bolt04_onion_key(key_type: bytes, secret: bytes) -> bytes:
     if key_type not in (b'rho', b'mu', b'um', b'ammag', b'pad', b'blinded_node_id'):
@@ -289,20 +294,19 @@ def calc_hops_data_for_payment(
     """
     if len(route) > NUM_MAX_EDGES_IN_PAYMENT_PATH:
         raise PaymentFailure(f"too long route ({len(route)} edges)")
-    # payload that will be seen by the last hop:
     amt = amount_msat
     cltv_abs = final_cltv_abs
+    # payload that will be seen by the last hop:
+    # for multipart payments we need to tell the receiver about the total and
+    # partial amounts
     hop_payload = {
         "amt_to_forward": {"amt_to_forward": amt},
         "outgoing_cltv_value": {"outgoing_cltv_value": cltv_abs},
-    }
-    # for multipart payments we need to tell the receiver about the total and
-    # partial amounts
-    hop_payload["payment_data"] = {
-        "payment_secret": payment_secret,
-        "total_msat": total_msat,
-        "amount_msat": amt
-    }
+        "payment_data": {
+            "payment_secret": payment_secret,
+            "total_msat": total_msat,
+            "amount_msat": amt,
+        }}
     hops_data = [OnionHopsDataSingle(payload=hop_payload)]
     # payloads, backwards from last hop (but excluding the first edge):
     for edge_index in range(len(route) - 1, 0, -1):
@@ -360,6 +364,36 @@ class ProcessedOnionPacket(NamedTuple):
     next_packet: OnionPacket
     trampoline_onion_packet: OnionPacket
 
+    @property
+    def amt_to_forward(self) -> Optional[int]:
+        k1 = k2 = 'amt_to_forward'
+        return self._get_from_payload(k1, k2, int)
+
+    @property
+    def outgoing_cltv_value(self) -> Optional[int]:
+        k1 = k2 = 'outgoing_cltv_value'
+        return self._get_from_payload(k1, k2, int)
+
+    @property
+    def next_chan_scid(self) -> Optional[ShortChannelID]:
+        k1 = k2 = 'short_channel_id'
+        return self._get_from_payload(k1, k2, ShortChannelID)
+
+    @property
+    def total_msat(self) -> Optional[int]:
+        return self._get_from_payload('payment_data', 'total_msat', int)
+
+    @property
+    def payment_secret(self) -> Optional[bytes]:
+        return self._get_from_payload('payment_data', 'payment_secret', bytes)
+
+    def _get_from_payload(self, k1: str, k2: str, res_type: type):
+        try:
+            result = self.hop_data.payload[k1][k2]
+            return res_type(result)
+        except Exception:
+            return None
+
 
 # TODO replay protection
 def process_onion_packet(
diff --git a/electrum/lnpeer.py b/electrum/lnpeer.py
index dc40eb0..faa32a9 100644
--- a/electrum/lnpeer.py
+++ b/electrum/lnpeer.py
@@ -2093,29 +2093,23 @@ class Peer(Logger, EventListener):
         Perform checks that are invariant (results do not depend on height, network conditions, etc).
         May raise OnionRoutingFailure
         """
-        try:
-            amt_to_forward = processed_onion.hop_data.payload["amt_to_forward"]["amt_to_forward"]
-        except Exception:
+        assert processed_onion.are_we_final, processed_onion
+        if (amt_to_forward := processed_onion.amt_to_forward) is None:
             log_fail_reason(f"'amt_to_forward' missing from onion")
             raise OnionRoutingFailure(code=OnionFailureCode.INVALID_ONION_PAYLOAD, data=b'\x00\x00\x00')
-
-        exc_incorrect_or_unknown_pd = OnionRoutingFailure(
-            code=OnionFailureCode.INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS,
-            data=amt_to_forward.to_bytes(8, byteorder="big")) # height will be added later
-        try:
-            cltv_abs_from_onion = processed_onion.hop_data.payload["outgoing_cltv_value"]["outgoing_cltv_value"]
-        except Exception:
+        if (cltv_abs_from_onion := processed_onion.outgoing_cltv_value) is None:
             log_fail_reason(f"'outgoing_cltv_value' missing from onion")
             raise OnionRoutingFailure(code=OnionFailureCode.INVALID_ONION_PAYLOAD, data=b'\x00\x00\x00')
-
         if cltv_abs_from_onion > htlc.cltv_abs:
             log_fail_reason(f"cltv_abs_from_onion != htlc.cltv_abs")
             raise OnionRoutingFailure(
                 code=OnionFailureCode.FINAL_INCORRECT_CLTV_EXPIRY,
                 data=htlc.cltv_abs.to_bytes(4, byteorder="big"))
-        try:
-            total_msat = processed_onion.hop_data.payload["payment_data"]["total_msat"]  # type: int
-        except Exception:
+
+        exc_incorrect_or_unknown_pd = OnionRoutingFailure(
+            code=OnionFailureCode.INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS,
+            data=amt_to_forward.to_bytes(8, byteorder="big")) # height will be added later
+        if (total_msat := processed_onion.total_msat) is None:
             log_fail_reason(f"'total_msat' missing from onion")
             raise exc_incorrect_or_unknown_pd
 
@@ -2132,9 +2126,7 @@ class Peer(Logger, EventListener):
                 code=OnionFailureCode.FINAL_INCORRECT_HTLC_AMOUNT,
                 data=htlc.amount_msat.to_bytes(8, byteorder="big"))
 
-        try:
-            payment_secret_from_onion = processed_onion.hop_data.payload["payment_data"]["payment_secret"]  # type: bytes
-        except Exception:
+        if (payment_secret_from_onion := processed_onion.payment_secret) is None:
             log_fail_reason(f"'payment_secret' missing from onion")
             raise exc_incorrect_or_unknown_pd
 
diff --git a/electrum/lnworker.py b/electrum/lnworker.py
index b0850c0..6b9b073 100644
--- a/electrum/lnworker.py
+++ b/electrum/lnworker.py
@@ -3456,18 +3456,11 @@ class LNWallet(LNWorker):
         chain = self.network.blockchain()
         if chain.is_tip_stale():
             raise OnionRoutingFailure(code=OnionFailureCode.TEMPORARY_NODE_FAILURE, data=b'')
-        try:
-            _next_chan_scid = processed_onion.hop_data.payload["short_channel_id"]["short_channel_id"]  # type: bytes
-            next_chan_scid = ShortChannelID(_next_chan_scid)
-        except Exception:
+        if (next_chan_scid := processed_onion.next_chan_scid) is None:
             raise OnionRoutingFailure(code=OnionFailureCode.INVALID_ONION_PAYLOAD, data=b'\x00\x00\x00')
-        try:
-            next_amount_msat_htlc = processed_onion.hop_data.payload["amt_to_forward"]["amt_to_forward"]
-        except Exception:
+        if (next_amount_msat_htlc := processed_onion.amt_to_forward) is None:
             raise OnionRoutingFailure(code=OnionFailureCode.INVALID_ONION_PAYLOAD, data=b'\x00\x00\x00')
-        try:
-            next_cltv_abs = processed_onion.hop_data.payload["outgoing_cltv_value"]["outgoing_cltv_value"]
-        except Exception:
+        if (next_cltv_abs := processed_onion.outgoing_cltv_value) is None:
             raise OnionRoutingFailure(code=OnionFailureCode.INVALID_ONION_PAYLOAD, data=b'\x00\x00\x00')
 
         next_chan = self.get_channel_by_short_id(next_chan_scid)